1. Field of the Invention
One disclosed aspect of the embodiments relates to a photoelectric conversion apparatus and an imaging system.
2. Description of the Related Art
An AF sensor described in Japanese Patent Laid-Open No. 2000-78472 is an example of a technology in an output of a photoelectric conversion unit is input to a differential amplifier circuit, an output of the differential amplifier circuit is input to a memory and a voltage follower circuit (hereinafter, called a follower circuit), and an output of the voltage follower circuit is input to an inverting input terminal of the differential amplifier circuit. In recent years, auto-focus performance in low luminance has gained attention. An AF sensor under low luminance increases the signal-to-noise (S/N) ratio of signal by increasing an electric charge conversion coefficient to relatively reduce an influence of circuit noise or accumulating an optical signal of several hundred ms, that is a relatively long time to gather many electric charges. In general, signal deterioration under low luminance in long-period accumulation may be caused by dark current noise and a minute leakage current of a circuit. In order to reduce such dark current noise, it is effective that each pixel is configured with an embedded photodiode structure and a transfer MOS structure. However, because an AF sensor monitors an amount of electric charges accumulated in a photodiode at all times for implementing an AGC function, it is required to directly connect the photodiode and a readout circuit and reset switch to continuously transfer the generated photocarriers to the readout circuit. In this case, because a depletion layer of the photodiode is in contact with a silicon interface, a signal in a long-period accumulation period under low luminance is influenced by dark current noise due to crystal lattice defects and impurities on the silicon surface. A signal in a long-period accumulation period under low luminance is further influenced by a leakage current when a reset switch normally including a MOS transistor is turned off. A leakage current when a reset switch is turned off is not only a source-drain leakage current called a sub-threshold leakage current. A drain-well leakage current dependent upon a gate-source voltage called Gate Induced Drain Leakage (hereinafter, called GIDL) may be a leakage current caused when a reset switch is turned off and may need to be taken into consideration.
A trade-off between a circuit operation range and a leakage current due to GIDL described in Japanese Patent Laid-Open No. 2000-78472 will be described. A relationship between a source potential and a drain current with a fixed gate potential in the vicinity of a sub-threshold region of a MOS transistor will be described. A sub-threshold leakage current decreases as a source potential decreases, but the leakage current of the GIDL increases with a certain level of potential or lower. As in Japanese Patent Laid-Open No. 2000-78472, in a case where the potential of a photodiode changes while accumulating photocarriers, the difference between the initial potential of the photodiode and a leakage tolerant potential of the MOS transistor corresponds to an operation voltage range. In other words, there occurs a tradeoff between a wider operation voltage range and an increase of GIDL leakage current.
A method may be considered which uses an accumulation circuit for a readout circuit in order to acquire a wider operation voltage range of a photodiode and suppress a leakage current due to GIDL by reducing the gate-source potential difference when a reset MOS transistor is turned off. For example, use of an accumulation circuit may be considered which switches the integral capacitance denoted by the reference numeral 30m in FIG. 2 in Japanese Patent Laid-Open No. 2005-321313.
Referring to Japanese Patent Laid-Open No. 2005-321313, FIG. 2, when switches SW31 and SW32 are turned off to perform long-period accumulation for increasing an electric charge conversion coefficient, the drain voltage fluctuation caused by a leakage current due to GIDL occurring in the switch SW32 may fluctuate the potential of the output terminal of a photodiode through the capacitance C32. In other words, because the number of positions where a leakage current due to GIDL occurs is increased to two, that is, the switch SW31 and the switch SW32, the use of an accumulation circuit may cancel the effect of a reduced gate-source potential difference when a reset MOS transistor is turned off.